Ethnic differences in breast cancer survival in New Zealand: contributions of differences in screening, treatment, tumor biology, demographics and comorbidities

We investigated the breast cancer survival disparity between Indigenous MAori and non-Indigenous European women in New Zealand, and quantified the relative contributions of patient, tumor and healthcare system factors toward this disparity. All women diagnosed with breast cancer in Waikato, New Zealand, during 1999-2012 were identified from the Waikato Breast Cancer Register. Cancer-specific survivals were compared using Kaplan-Meier survival curves, while contributions of different factors toward the survival disparity were quantified with serial Cox proportional hazard modeling. Of the 2,679 women included in this study, 2,260 (84.4 %) were NZ European and 419 (15.6 %) were MAori. Compared with NZ European women, MAori women had a significantly higher age-adjusted cancer-specific mortality (HR 2.02, 95 % CI 1.59-2.58) with significantly lower 5-year (86.8 vs. 76.1 %, p < 0.001) and 10-year (79.9 vs. 66.9 %, p < 0.001 %) crude cancer-specific survivals. Stage at diagnosis made the greatest contribution (approximately 25-40 %), while screening, treatment and patient factors (i.e., comorbidity, obesity and smoking) contributed by approximately 15 % each toward the survival disparity. The final model accounted for almost all of the cancer survival disparity (HR 1.07, 95 % CI 0.80-1.44). MAori women experience an age-adjusted risk of death from breast cancer, which is more than twice that for NZ European women. Equity-focussed improvements in health care, including increasing mammographic screening coverage and providing equitable quality and timely cancer care, may improve the survival disparity between MAori and NZ European women.